The present invention pertains to silver catalysts intended for the manufacture of ethylene oxide by the oxidation of ethylene with molecular oxygen.
More particularly, it pertains to catalysts in which the silver is deposited on a porous refractory support which is modified by the presence of at least one promoter deposited on the said support, and to the preparation of such catalysts and their use for manufacturing ethylene oxide.
The supports used are most often formed of alumina, silica-alumina, magnesia, pumice, zirconia, clay, ceramic, natural or artificial graphite, asbestos, natural or artificial zeolite, or silicon carbide, in various forms, such as in the form of extruded parts, tablets, rings, spheres, and the like, with external dimensions most often comprised between 2 mm and 10 mm.
The preferred supports have a small specific surface, generally equaling at most 10 m.sup.2 /g and an increased porosity, with the pore volume often exceeding 20% and being able to reach up to 60%.
In most cases, the specific surface is determined by the nitrogen adsorption method or BET method described by Brunauer, Emmet and Teller in The Journal of American Chemical Society 60, 309 (1938), while the porosity is measured by the mercury porosimetric method recommended by E. W. Washburn in Proceedings of the National Academy of Sciences of the USA 7, 115 (1927) and described by L. C. Darke in Industrial and Engineering Chemistry 41, 780 (1949) and in Industrial and Engineering Chemistry, Analytical Edition 17, 782 (1945).
Among the supports possessing the above-mentioned characteristics, those used most often are of the type of alumina of particularly high purity and alumina with low silica content. Alpha-alumina is particularly suitable.
The selection of such a preferred support does not suffice in itself, because the activity, the selectivity for ethylene oxide, and the long-term behavior of the catalysts are dependent not only on the support, but also on the nature of the precursors of the active constituents and the technique employed to deposit the silver and the promoters on the support.
It is a known fact that two techniques are currently in use: coating of the support with the precursors of the active phases suspended in a liquid, and impregnation of the support with the said precursors being used in a solubilized form.
It is commonly admitted at present that the second technique is more favorable than the first one, particularly because of the stronger adhesion of the active constituents.
The prior art usually qualifies the impregnation in which the silver is always accompanied with the promoter during the deposition operation as "simultaneous" impregnation, and the impregnation in which the total amount of the silver is deposited completely separated from the deposition of the promoter as "sequential" impregnation.
As is shown; for example, by U.S. Pat. Nos. 3,563,914, 3,962,136, 4,033,903, 4,010,115, 4,012,425, and 4,207,210 or Belgian Pat. No. 793,658, the sequential impregnation is practically equivalent to the simultaneous impregnation if the deposition of the promoter precedes that of the silver but is clearly inferior to it in the opposite case.
According to European Patent Application published as U.S. Pat. No. 0076504, not even the transformation of the silver deposited into metallic silver prior to the deposition of the promoter is sufficient to cause the sequential impregnation to be superior to the simultaneous impregnation.
Even though improvement of the catalysts, and even minor improvements in view of their considerable economic significance within the framework of a major industrial process for the larger-scale production of ethylene oxide, have always been a concern in industry, the total amount of the silver and the total amount of the promoter have continued to be deposited on the support according to the impregnation technique carried out either completely simultaneously or completely separately.
There was no reason to abandon this practice, because even though the two methods, simultaneous deposition and sequential deposition, are totally different from one another, they are ultimately functionally equivalent.